1. Field of the Invention
The present invention relates to an antenna duplexer that separates a transmit signal and a receive signal.
2. Description of the Related Art
The recent development of mobile communication system has realized rapid popularization of mobile phones, portable information terminals and the like. In the field of the mobile phone, multiband and multimode systems are rapidly spreading, and besides accessorial wireless interface functions such as wireless LAN, Bluetooth, or GPS are being added one after another. Such situation is provoking a strong demand for further reduction in dimensions and higher integration level of RF circuits in the mobile phone and hence, in turn, a greater demand is arising for reduction in dimensions, lower cost yet higher performance of an antenna duplexer, which is one of the major components of the RF circuit.
The antenna duplexer is employed for separating a transmit signal and a receive signal of different frequencies, and includes a filter for transmission and a filter for reception. These filters often include a one terminal-pair resonator shown in FIG. 17. As to the resonator, large-sized dielectric resonators which have been widely used are recently being replaced with small-sized piezoelectric thin film resonators or surface acoustic wave resonators. In addition, boundary wave resonators have lately been developed. The structure of the resonators will be described hereunder.
FIG. 18(a)-(b) illustrates the basic structure of the piezoelectric thin film resonator. FIG. 18(a) is a plan view of the piezoelectric thin film resonator 500, and FIG. 18(b) is a cross-sectional view taken along the line II-II in FIG. 18(a).
The piezoelectric thin film resonator 500 includes a lower electrode layer 502, a piezoelectric layer 503, and an upper electrode layer 501 stacked over a hollow 505 formed in a substrate 504, made of silicon for example. The upper electrode layer 501 serves as an input terminal 506, and the lower electrode layer 502 serves as an output terminal 507. The piezoelectric layer 503 is constituted of, for example, aluminum nitride. The hollow 505 may be formed by perforating a through hole from the lower surface of the substrate 504 as shown in FIG. 18(b), or may be a cavity formed on the surface of the substrate 504 utilizing a sacrificial layer.
FIG. 18(c)-(d) illustrates the basic structure of another piezoelectric thin film resonator. FIG. 18(c) is a plan view of the piezoelectric thin film resonator 510, and FIG. 18(d) is a cross-sectional view taken along the line III-III in FIG. 18(a).
The piezoelectric thin film resonator 510 includes an acoustic multilayer 515 (hereinafter, simply “multilayer”) constituted of high acoustic impedance layers and low acoustic impedance layers alternately stacked, in place of the hollow 505 in the piezoelectric thin film resonator 500, and a lower electrode layer 512, a piezoelectric layer 513, and an upper electrode layer 511 are stacked over the multilayer. The resonant frequency of the piezoelectric thin film resonator 510 is determined by the thickness of the multilayer 515 and propagation speed of vertical vibration in a thicknesswise direction.
FIG. 19(a)-(b) illustrates the basic structure of the surface acoustic wave resonator. FIG. 19(a) is a plan view of the surface acoustic wave resonator 520, and FIG. 19(b) is a cross-sectional view taken along the line IV-IV in FIG. 19(a).
The surface acoustic wave resonator 520 includes interdigital transducers (hereinafter abbreviated as IDT) 521 located on a piezoelectric substrate 524 and connected to an input terminal 526 and an output terminal 527, and reflectors 522 located on the respective sides of the IDT 521. The IDT 521 and the reflector 522 are formed on a metal, for example aluminum (Al). FIGS. 19(a) and 19(b) depict fewer numbers of the electrode teeth of the reflector 522 and the IDT 521, than what they really are. The resonant frequency of the surface acoustic wave resonator 520 is determined by the electrode pitch of the IDT 521 and propagation speed of the surface acoustic wave.
FIG. 20(a)-(b) illustrates the basic structure of the boundary wave resonator. FIG. 20(a) is a plan view of the boundary wave wave resonator 530, and FIG. 20(b) is a cross-sectional view taken along the line V-V in FIG. 20(a).
The boundary wave wave resonator 530 has a similar basic structure to that of the surface acoustic wave resonator 520, and the difference is that the former includes two types of dielectric layers 538, 539 over IDT 531 and a reflector 532. The resonant frequency of the boundary wave resonator 530 is determined by the electrode pitch of the IDT 531 and propagation speed of the boundary wave.
Hereunder, the filter for transmission or for reception will now be described.
FIG. 21 is a circuit diagram for explaining a ladder-type filter employed in the filter for transmission or reception.
The ladder-type filter 540 includes a plurality of the foregoing one terminal-pair resonator, connected so as to form a serial arm and a parallel arm, and is widely used as the filter for transmission or reception. The ladder-type filter 540 has the advantage that the bandwidth can be broadened with relatively low loss and high attenuation can be attained in the vicinity of the passing band, and of high power-withstanding capability. In addition, a longitudinal mode coupled filter is also widely employed. The longitudinal mode coupled filter generally includes, as basic structure, a double mode type surface acoustic wave filter (DMS: Double Mode SAW) as shown in FIGS. 22 and 23.
FIG. 22 is a schematic diagram for explaining the double mode type surface acoustic wave filter of an unbalanced type.
The unbalanced double mode type surface acoustic wave filter 550 includes a plurality of input IDTs 551 and output IDTs 553 provided on a piezoelectric substrate (not shown), and reflectors 552 located on an outer side of the IDTs. The double mode type surface acoustic wave filter 550 can constitute a filter having excellent attenuation over a broad bandwidth and a balanced/unbalanced conversion function.
FIG. 23 is a schematic diagram for explaining the balanced/unbalanced-convertible double mode type surface acoustic wave filter.
The balanced/unbalanced-convertible double mode type surface acoustic wave filter 560 includes an unbalanced type input terminal and a balanced type output terminal. In the filter 560, one of the output IDTs 563′ is oriented opposite to the other output IDT 563, so as to form the output terminal of the balanced type. Except for this aspect, the double mode type surface acoustic wave filter 560 is the same as the unbalanced double mode type surface acoustic wave filter 550.
As described earlier regarding the boundary wave resonator 530, the double mode type filter can be used as a double mode type boundary wave filter, upon providing the two types of dielectric layers on the IDT.
The following passages describe the antenna duplexer. FIG. 24 is a block diagram for explaining a basic structure of the antenna duplexer.
The antenna duplexer 601 serves to separate a transmit signal and a receive signal of different frequencies. For such purpose, the antenna duplexer 601 includes a transmit filter 602, a receive filter 603, a matching circuit 604, an antenna terminal 607, a transmit terminal 608, and a receive terminal 609. The transmit signal is input through the transmit terminal 608, passes through the transmit filter 602 and the matching circuit 604, and is output through the antenna terminal 607. The receive signal is input through the antenna terminal 607, passes through the matching circuit 604 and the receive filter 603, and is output through the receive terminal 609.
The matching circuit 604 is provided between the antenna terminal 607 and the two filters 602, 603, to prevent the increase of leak loss of the transmit and receive signal. Specifically, the matching circuit 604 increases the impedance of the transmission band in the receive filter 603, to thereby inhibit the transmit signal input through the transmit terminal 608 from deviating to the receive filter 603 and being output to the side of the receive terminal 609, and also increases the impedance of the transmit filter 602, to thereby inhibit the receive signal input through the antenna terminal 607 from deviating to the transmit filter 602 and being output to the side of the transmit terminal 608. Here, the antenna duplexer 601 is constituted of components integrated as a unit, such that the transmit filter 602, the receive filter 603 and the matching circuit 604 are accommodated in a package, and the antenna terminal 607, the transmit terminal 608 and the receive terminal 609 are provided on the outer periphery of the package.
With respect to the latest mobile phones, the demand for higher attenuation and higher isolation characteristic from the antenna duplexer has grown stronger than ever, owing to various factors such as the spread of multiband/multimode system, expanding variation of accessorial wireless interface functions, decreased use of interstage filters, and so forth.
To increase the out-of-band attenuation and improve the isolation characteristic, the layout of the ground pattern in the package plays a critical role. Generally, a large common ground is formed inside the package or on a surface where a footpad is provided (hereinafter, “footpad surface”), to thereby reinforce the ground.
FIG. 25 depicts an internal structure of an antenna duplexer disclosed in JP-B-3778902.
The antenna duplexer 701 includes a multilayer package 710 and a filter chip 740 face-down mounted therein. FIG. 26 is a circuit diagram of the antenna duplexer 701. In this conventional structure, the package 710 includes six layers, in which a plurality of ground patterns 770 serving as a large common ground is provided. Accordingly, the antenna duplexer becomes considerably large in dimensions, for example 3.8 mm×3.8 mm×1.4 mm (1.4 mm is the height).
With respect to the antenna duplexer, as already stated, reduction in dimensions including height is keenly required, in addition to higher performance. However, attempting to reduce the dimensions including the height of the package imposes difficulty in securing a sufficient space for arranging the ground pattern.
Moreover, in the field of semiconductor devices, equilibration of circuits in devices such as a mixer of a receiving circuit or a low noise amplifier (LNA) is being promoted, for improvement of noise characteristic, for example against a crosstalk between the devices. The antenna duplexer to be connected with such semiconductor devices is also required to have a balanced receive terminal. In the antenna duplexer having a balanced receive terminal, the balanced/unbalanced-convertible longitudinal mode coupled filter is often employed as the receive filter. The attenuation in the transmission band and isolation characteristic are quite susceptible to the layout of the ground pattern of the longitudinal mode coupled filter, and therefore special attention has to be paid to the layout of the ground pattern.